1. Field of the Invention
The invention relates to an optical switch which is switchable by means of light and comprises an optical wave-guiding structure whose refractive index is variable by means of light intensity, and at least an entrance to which a signal can be applied and at least an exit, said switch further comprising a radiation source unit whose radiation can be injected into the wave-guiding structure, the radiation source unit comprising at least a pulsed laser for supplying a pulse series having a repetition time T and a pulse duration p and a medium for transporting the radiation supplied by the laser.
The invention also relates to a transmitter and a receiver provided with such an optical switch and to a multiplex transmission system comprising such a transmitter and/or receiver.
2. Discussion of the Related Art
A switch of the type described in the opening paragraph is known, for example from the article "Non-linear Optical Loop Mirror" by N. J. Doran and D. Wood in Optics Letters, Vol. 13, No. 1, January 1988. The switch described in this article is implemented as a NOLM. The wave-guiding structure is a glass fibre ring which is obtained by interconnecting the two ends of a directional coupler, generally a 50:50 coupler, so that two gates functioning as an entrance and an exit are formed on the ring. When a signal is introduced into the ring via the entrance gate, this signal will be split into two equal signal portions by the directional coupler, which signal portions will traverse the ring in opposite directions and recombine in the coupler. Moreover, the ring accommodates a polarization-controlling element ensuring that the optical path lengths covered by the two signal portions remain equal. Destructive interference occurs at the exit gate and consequently no intensity will be measured at this gate. Using the optical non-linear property of a glass fibre, i.e. the refractive index is dependent on the optical intensity in this fibre in accordance with the relation n=n.sub.O +n.sub.2 I, in which n.sub.2 is the non-linear coefficient, an asymmetry may be caused in the ring due to injection of radiation which traverses the ring in one direction only. This radiation causes a phase change in the signal portion propagating in the same direction due to cross-phase modulation and will consequently produce a phase difference between the two signal portions propagating in opposite directions. The duration of the asymmetry caused and hence the duration of the resultant phase difference corresponds to the duration of the period in which extra radiation is injected. Due to this asymmetry, a different intensity distribution will temporarily occur at the two gates and the signal will be passed on to the exit gate. The absence or presence of a control pulse allows the ring to be switched between the mirror and the pass circuit.
A drawback of the known switch is that the rate at which the laser pulses succeed each other is limited by the repetition time of the laser. This repetition time is determined by the possibility of direct modulation of the diode laser and is of the order of 10 GHz. Moreover, the rate at which successive pulses are generated is limited by the presence of jitter which is produced during the build-up of a subsequent pulse, so that the pulse position is not sharply defined with respect to the pulse period.